libxml2: Set environment variables

[brian6932]

https://cmake.org/cmake/help/latest/module/FindLibXml2.html

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libxml2

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[Lutra-Fs]

I’d avoid setting LIBXML2_INCLUDE_DIR/LIBXML2_LIBRARY via env_set. Those are CMake cache variables, not broadly respected environment variables. A safer approach is to add notes that show users how to configure their own package manager like CMake, Meson, MSBuild etc.

[brian6932]

Those are CMake cache variables, not broadly respected environment variables.

What's the point of installing the package if you don't want to set the environment variables? This is a library package used for building with CMake and other generators. If someone wants to change the value, they're free to do that, but it should definitely be in the package.

[Lutra-Fs]

Those are CMake cache variables, not broadly respected environment variables.

What's the point of installing the package if you don't want to set the environment variables? This is a library package used for building with CMake and other generators. If someone wants to change the value, they're free to do that, but it should definitely be in the package.

What I mean is that you cannot assume user is using CMake generator, and we should not set all possible env var for user.

[brian6932]

we should not set all possible env var for user

I don't agree, it's only a benefit to set it, we're packaging a library. Why package it at all if you don't set the environment variables? All library packages set the relevant environment variables. We're not modifying the CMAKE_PREFIX_PATH here, just setting library-specific environment variables, this is standard practice.

Do you have any idea how many environment variables a developer would have to set if every single packaged library lacked necessary environment variables on large codebases? This is literally one of the largest reasons why people use vcpkg, conan, and linux distro package managers instead of manually downloading dependencies.

[Lutra-Fs]

Thanks for opening this and for thinking about downstream dev experience—totally with you on making library packages “just work.” As a developer myself, I appreciate when dependencies are easy to wire up.

That said, exporting LIBXML2_INCLUDE_DIR / LIBXML2_LIBRARY via env_set isn’t the right lever here. Those names are CMake cache variables used by FindLibXml2 at configure time, not environment variables that CMake consumes automatically. So setting them in the OS environment won’t reliably help CMake—and it won’t help Meson or MSBuild users either.

The portable discovery interfaces are:

• CMake: use LibXml2_ROOT (env or CMake variable) and/or CMAKE_PREFIX_PATH, then find_package(LibXml2) → LibXml2::LibXml2.
• Meson/pkg-config: add ...\lib\pkgconfig to PKG_CONFIG_PATH (semicolon-separated on Windows) and use dependency('libxml-2.0').

This approach matches the official docs and avoids global env pollution and version pinning issues across multiple installs/versions.

Suggestion: instead of env_set, add a short notes section showing the commands, so like for a user who only have one build system (e.g. MSBuild) installed (and not CMake) can also benefit with your change. It would also benefit if you update the other lib package in main bucket as well for c/cpp:

# CMake (either of these works)
$prefix = "$env:SCOOP\apps\libxml2\current"
cmake -S . -B build -DLibXml2_ROOT="$prefix"
# or
$env:CMAKE_PREFIX_PATH = "$prefix;$env:CMAKE_PREFIX_PATH"
cmake -S . -B build

# Meson / pkg-config
$prefix = "$env:SCOOP\apps\libxml2\current"
$env:PKG_CONFIG_PATH = "$prefix\lib\pkgconfig;$env:PKG_CONFIG_PATH"
# meson setup build   # then dependency('libxml-2.0')

References:

• Mastering CMake — CMake Cache: https://cmake.org/cmake/help/book/mastering-cmake/chapter/CMake%20Cache.html
• <PackageName>_ROOT (e.g., LibXml2_ROOT): https://cmake.org/cmake/help/latest/envvar/PackageName_ROOT.html
• FindLibXml2 module: https://cmake.org/cmake/help/latest/module/FindLibXml2.html
• Meson & pkg-config: https://mesonbuild.com/Pkgconfig-module.html

[brian6932]

I don't think we should be modifying package config path, or CMake prefix path, you're generally not supposed to use package config on Windows. Sure, I wouldn't be opposed to adding LibXml2_ROOT, but you should be setting cache variables, they literally get checked:
https://github.com/search?q=repo%3AKitware%2FCMake+LibXml2_&type=code
It doesn't prevent CMake from re-setting them again, or the user doing so, I set these as someone using the library, so I know it works. The cache article tells you not to edit the CMakeLists.txt file, nothing against setting the environment variables.